Aggregate is a particulate material used in the construction industry in a wide variety of applications including foundation bases, road bases, drainage culverts, railroad beds, and concrete filler. According to their density, aggregates may be categorized as “normal-weight aggregate” (e.g. granite, basalt, etc.), and “lightweight aggregate” (e.g. pumice, volcanic cinders, etc.). Typically, normal weight aggregate is obtained from mining combined with a crushing operation to produce particulates of a desired size. However, mining operations can disrupt the local environment and mines are often located far from population centers that require large quantities of aggregate.
With increasing concern over the excessive exploitation of natural aggregates, synthetic aggregate, including also both normal-weight and lightweight aggregate, has been explored as a new source of aggregate material. Some synthetic aggregates are classified as lightweight aggregates with a loose bulk density lower than 1200 kg/m3. Lightweight aggregates are particularly attractive for the production of lightweight blocks and other lightweight construction products. The additional benefits of these products are their superior thermal and acoustic insulation performance.
Synthetic lightweight aggregates are now mainly manufactured by sintering expanding clays, shale, etc. The general steps of producing sintered lightweight aggregates are mixing of raw materials such as clays with water and additives to allow a formation of desired spherical pellet sizes through an agglomeration process. The spherical pellets are then fed directly into a rotary kiln (or sinter strand) at temperatures in the range of 1100-1200° C. Lightweight solid pellets are then produced by particle fusion and structure bloating. However, this energy intensive process and accompanying emission of greenhouses gases resulting from the burning of fuel to run a kiln raises environmental concerns. It is estimated that about 0.3 ton of CO2 is produced for manufacturing one ton of lightweight aggregate.
At the same time that large population centers have a need for construction aggregate, these same population centers produce large quantities of waste that are currently disposed of in costly landfills. Such waste includes sewage sludge ash produced from incineration of sewage and construction waste including demolished concrete. Through a series of recycling processes, demolished concrete may be turned into recycled aggregates of different sizes for use in civil engineering applications. However, during the recycling processes, a significant amount of unusable concrete fines (20% or more) are produced, and this fraction of concrete fines is prohibited for use in concrete, thus finding a way to utilize it has become a priority in managing construction and demolition waste.
There is a need in the art for synthetic aggregate material produced from recycled waste products, particularly in large urban locations that are both sources of waste and consumers of aggregate.